So the problem started with difficulty cold starting. Engine would fire and then die. If I gave it throttle I could keep it running but if I let off it would die, and wouldn't fire for a few minutes. Place reeked of gas, and would spit black stuff out of the tailpipe. Once it got up to operating temperature it was fine.
Replaced the coolant temp sensor and its better but still having trouble.
Pulled the plugs and number 4 was all black, rest looked fine. Cleaned out intake to turbo tube (gunk from pcv valve), air filter and reoiled, and throttle body (little bit of carbon build up. Checked distributer cap which looked fine, and replaced plugs. Test drive and 4 is black again after like 15 min of varied driving. Swapped plugs 1 and 4 and injectors from cyl 3 and 4. Test drive and and cyl 4 still black.
One of the times I pulled the plugs it was a little bit of oil. Have not seen it again.
Ran a compression test (operating temp and WOT) and got in order.
120 120 130 137
When I cold started today prior to compression test it started, and within 30 seconds died. Wideband was reading 9:1 then died. Gauge read 12.1 after dying, and wouldn't fire. Gave it two minutes, tried again and back at 9:1. After giving a blip on the throttle went up to 11:1 then slowly climbed to 14.7 as it warmed up. Left that lovely black mark on the garage floor posted earlier. Once I got it in the road no more problems. Made 4 stops, one letting engine cool for an hour and no problems.
Afr stays at 14.7 ish while idling and 12.5 ish under boost.
Not sure what to do next, any suggestions for what to check?
TLDR
Cyl 4 spark plug is black. Engine rich on cold start. Blowing black stuff out tailpipe. Not smoking. Reeks of gas on cold start
Injectors ok
Plugs ok
Cap ok
Compression higher than others
Coolant temp sens replaced